Microwave dynatron



Feb. 12, 1952 A. M. SKELLETT ETAL MICROWAVE DYNATRON Filed June 50, 1949 f NP f mm m mam Z M m T m rm /A 1, M w W a W .1 d f f Patented Feb. 12, 1952 MICROWAVE DYNATRON Albert M. Skellett, Madison, and Lawrence E. Loveridge, Rutherford, N. J., assignors to National Union Radio Corporation, Orange, N. J a corporation of Delaware Application June 30, 1949, Serial No. 102,172

8 Claims.

This invention relates to electron discharge tubes and more particularly to tubes of the dynatron type.

A principal object of the invention is to provide an improved dynatron tube for use at microwave or. superhigh frequencies.

Another object is to provide a micro -wave dynatron having an electrode array for enabling the tube to act as an eflicient superhigh frequency oscillator while effecting a substantial reduction of spurious or unwanted frequencies.

A feature of the invention relates to a dynatron tube having an electron emitter of the electron gun type, a dynode or secondary electron emission target, and a special grid construction located between the emitter and target for setting up a field-free region adjacent the target to prevent the generation of spurious oscillations such as those of the Barkhausen-Kurz type.

Another feature relates to an electron tube of the dynatron type having a Faraday cage located between the control grid and the dynode to improve the operating eiliciency and frequency characteristics of the tube.

A further feature relates to a novel grid arrangement for dynatrons, whereby the life of the dynatron is greatly increased without reducing the efilciency or stability of the oscillation generation, while at the same time reducing spurious oscillations.

A further feature relates to the novel organization, arrangement and relative location and interconnection of parts which cooperate to provide an improved micro-wave dynatron.

In the drawing which shows, by way of example, one preferred embodiment,

Fig. 1 is a view, partly sectional, of a dynatron tube according to the invention.

Fig. 2 is a typical schematic wiring diagram for use with the tube of Fig. 1.

As is well-known, the dynatron type of electron tube oscillator employs an electron-emitting cathode, a secondary electron-emissive target, and an intervening grid which is positive with respect to the target and cathode. By means of a tuned oscillatory circuit connected between the target and cathode, and by suitable choice of electrode spacings and applied voltages, the tube acts as a negative resistance oscillator. When they dynatron is operating, the target must be bombarded with primary electrons from the cathode at sufl'lcient velocity to release secondary electrons from the target. In order to increase the ratio of secondary electrons to primary electrons, it has been the practice either to coat the dynode surface with a special secondary electron emission material, or to use a special material for the dynode itself. In either case, the continuous bombardment of the dynode tends to reduce the useful life of the tube and tends to render it unstable as a frequency generator.

Furthermore, in order to maintain proper operation as a dynatron, it is important that the target be protected against deposition thereon of any of the cathode coating material, which deposition may tend to occur because of the relatively high temperature at which the cathode operates, and because of the relatively close spacing which has heretofore been employed between the cathode and target. It has been proposed heretofore to prevent this undesirable deposition of cathode material on the dynode by providing shields or barriers adjacent the cathode. We have found that it is more advisable to rely upon a greater spacing between the cathode and target and to provide a special electrode array so that this increased spacing does not cut down the desired quantity of primary electrons reaching the dynode from the cathode.

The present invention, therefore, overcomes the above-noted and other disadvantages ordinarily present in a dynatron tube, by employing as the primary electron source a concentrated electron stream or beam of electrons of the desired velocity such as are ordinarily provided by the so-called electron guns employed in cathode ray tubes, and spacing the dynode a substantial distance from the cathode. However, in order to maintain the operating efficiency, we have found it advisable to provide in the region between the dynode and cathode, means to set up a field-free space. This is accomplished, according to the invention, by using a dual grid construction in the form of a Faraday cage between the dynode and cathode. Because of the comparatively great spacing between the dynode and cathode, any electrons which tend to oscillate through or with respect to the dynode-grid i. e., the grid adjacent the dynode, by reason of the negative gradient field from the cathode, are collected within the Faraday cage before they can oscillate to any degree with respect to the said dynode-grid.

Referring to Fig. l,-the tube according to the invention comprises an evacuated enclosing glass bulb or envelope I, through the cylindrical wall of which are sealed a pair of fiat metal rings 2, 3. In the well-known manner, these rings are made of a suitable alloy, such for example as Kovar, which provides a vacuum-tight seal to the glass I. Fastened to the inner margin of disc 2, by suitable rivets or screws 4, is the flat flanged portion of a dish-shaped metal member 6. Member 6 may be of copper or any other material which acts as an eflicient secondary electron emitter when its surface is bombarded by primary electrons. Member 6 is provided with a vided with a comparatively large opening across Y which is fastened a mesh grid or foraminous plate ll. Preferably, the members l0 and II are of copper or similar metal. The lower end of memher I 0 also carries a closure plate l2 having a smaller central opening l3 through which the primary electron stream is arranged to pass to the target or dynode 6. The dynode 6 has connected thereto a suitable lead-in wire l4 which is vacuum-tight sealed through the top wall of the bulb l.

Suitably supported from the lower end of bulb l is a cathode member I5 which may be in the form of a tubular metal sleeve having on its interior, but electrically insulated therefrom, any well-known heater element It for raising the upper fiat end W of the sleeve i5 to a suitable electron-emitting temperature. The upper end ii is coated on its outer face with any suitable electron-emissive material such as a mixture of the alkaline earth oxides. This type of cathode is well-known in the electron gun and cathode ray tube arts. It will be understood, of course, that the heater element i6 is provided with appropriate current lead-ins (not shown) which are sealed through the bottom wall of bulb i. Like- Wise, 2. separate lead-in id is sealed through the bulb wall and is connected to the cathode sleeve i5. It will also be understood that the bulb l is provided with the usual sealed-0d glass tubulation l9 and through which the bulb has been evacuated prior to sealing-ofl'.

Also suitably mounted at the lower end of the bulb I is an inverted cup-shaped metal member which is attached to the support wires and one of which may be connected to a corresponding lead-in 22. The upper end of member 20 is provided with an enlarged opening which is coaxial with respect to the cathode sleeve. Welded or otherwise fastened to the flanged portion 22a is a metal disc 23, preferably of nickel or some similar metal and having a central opening 23a in alignment with the center of the cathode end and in coaxial alignment with the opening l3 and the center of the target 6. Suitably fastened between the disc 23 and the disc I 2 is a layer of insulation 24 such as mica, likewise having a central opening 25 corresponding in size to the opening l3 and in alignment therewith. By this arrangement it is possible to apply different direct current potentials to the disc 23 which acts in the nature of a control grid for the electrons emerging from the electron gun cathode. Likewise, it is possible by means of the annular ring 3 to apply a different potential to the structure l0, and therefore to the grid I I; while the target 6 can be connected into the oscillatory circuit separately by the ring 2. A diagram of typical electrical connections for the various electrodes is shown in Fig. 2, wherein the parts which are identical with those of Fig. 1, bear the same biassed by means of a suitable direct current bias potential 26 to control the intensity and/or velocity of the electron beam from the gun cathode ll. The Faraday cage member I0 is then connected to a suitable high potential tap 21 on a direct current supply source 28 the negative end of which is connected to cathode l1. The dynode 6 is connected through a suitable tuned oscillatory circuit represented schematically by the inductance 29 and capacitance 30 to a point on the supply source 28 which is less positive than that to which the cage I0 is connected. With this arrangement, therefore, the space between the grid II and the dynode 6 has a negative resistance characteristic as is well-known in the dynatron art. Since the electrons are emitted from the electron gun cathode in the form of a concentrated beam or stream, they pass through the grid II and bombard the dynode 6. Under ordinary circumstances, some of the secondary electrons that would be emitted from the dynode 6, might pass through the grid II in a reverse direction towards the cathode l1. Ordinarily, this cathode I! would be relatively close to the grid II, and its negative potential gradient would cause these relatively slow-moving secondary electrons to reverse again their direction of movement through the grid Ill towards the dynode 6. This oscillation of the secondary electrons through or with respect to the grid H, would ordinarily set up spurious or undesired oscillations. However, by reason of the fact that, according to the invention, there is a comparatively long space between the dynode 6 and the cathode ill, and since this space is occupied to the greatest extent by the Faraday cage it, these reversely-moving secondary electrons are collected on the interior of the cage Ill before they have a chance to reverse their direction through grid M towards the dynode. In other words, the member It provides a field-free space inside the cage to prevent the secondary electrons from oscillating with respect to the dynode-grid ll.

Not only does the member ID serve to eliminate these spurious oscillations, but it also serves other purposes. Thus. it acts as a physical barrier to a certain extent, to prevent the deposition of cathode material on the target 6, and it permits the target to be spaced a substantial distance from the cathode, for example as much as a centimeter or more. The member l0 therefore performs in itself three important functions. First, it provides the necessary support for the dynode-grid ll; second, it protects the dynode against a certain amount of undesired deposition of cathode material thereon; third, it permits greater spacing between the dynode and cathode without any danger of undesirable reduction in the quantity of primary electrons reaching the dynode. In addition to these three functions, by reason of the extended length of the member In and its relatively large diameter, it provides a substantial heat radiating surface for the dynode-grid II. It has been found that in ordinary dynatron tubes where the dynodegrid is relatively close to the dynode and without the cage construction It! as described, the existence of large secondary emission currents from the dynode causes this grid to become white hot. Since the dynode-grid is relatively close to the dynode surface, the heat of this grid would ordinarily tend to ruin that surface. However,

aseaaeo because of the relatively large size of the member l0, these large secondary emission currents are distributed over a relatively large area, and the dynode-grid ll remains comparatively cool.

While one particular embodiment has been disclosed herein, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A dynatron tube, comprising an evacuated enclosing envelope containing an electronemitting cathode, a dynode, a dynode-grid adjacent the dynode and positively biased with respect thereto to set up a negative resistance therebetween, and Faraday cage means constituted in part of said dynode-grid to set up between the grid and cathode a field-free space for secondary electrons which tend to oscillate with respect to said grid.

2. A dynatron tube, comprising an evacuated envelope containing an electron-emitting cathode, a dynode, a dynode-grid adjacent the dynode and positively biassed with respect thereto to set up a negative resistance therebetween, and electrode means between said grid and cathode and forming with said grid a Faraday cage.

3. An electron tube of the type described, comprising an evacuated envelope containing a dynode, an electron gun cathode for projecting a stream of electrons to said dynode, a grid member adjacent said cathode for controlling the intensity of the electron stream from the cathode, said grid member including a perforated metal disc, a second perforated metal disc adjacent said first disc on the side facing the dynode but insulated from the first disc, said discs having their perforations aligned with said cathode, a tubular metal member attached at one end to said second disc, and a dynode-grid mounted across the opposite end of said tubular metal member adjacent said dynode, said second disc forming with said tubular member and with said dynode-grid a Faraday cage.

4. An electron tube according to claim 3, in which said tubular member is provided with a separate sealed lead-in member for applying to said dynode-grid a higher positive potential than is applied to said dynode, and separate lead-in means are provided for applying an intensity bias potential to said controlling grid member.

5. A dynatron tube, comprising an evacuated glass envelope, a first annular metal lead-in 6 member sealed through the wall of said envelope, another annular metal lead-in member also sealed through the wall of the envelope, a dynode attached to the inner margin of said first annular member, a Faraday cage attached to the inner margin of said second annular member, said Faraday cage having one end wall thereof adjacent said dynode and bridged by a dynode-grid, the opposite end of said Faraday cage being bridged by a centrally perforated metal disc, an electron-emitting cathode having an emitting surface in alignment with said central perforation, and an electron contro1 grid having a central perforation also in alignment with said cathode and located between said cathode and said perforated disc.

6. A tube according to claim 5, in which an insulator disc is provided between saidcontrol grid and said perforated disc.

7. A dynatron tube, comprising an evacuated enclosing bulb, a tubular metal cathode sleeve having a substantially flat end provided with a coating of electron-emissive material, a heater element within said sleeve, a control grid for said cathode in the form of an inverted metal cup having a centrally perforated flat bottom wall with its perforation in alignment with said flat end of said cathode, a Faraday cage comprising a centrally perforated fiat disc at one end and a dynode-grid at the other, means supporting said cage with said perforated disc closely adjacent to but insulated from said control grid, an annular metal member sealed through the wall of said bulb for supporting said Faraday cage, and a dynode supported Within said envelope adjacent said dynode grid.

8. A dynatron according to claim 7, in which a second annular metal member is also sealed through the wall of said bulb for supporting said dynode.

ALBERT M. STT. LAWRENCE E. LOVERIDGE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA 3 

